Recently, planar light-emitting bodies used as backlights for various types of displays, display boards (such as signboards and emergency lights), light sources for lighting fixtures and the like have received attention for its excellent features such as high brightness, high luminous efficiency, small thickness and light weight. Among the planar light-emitting bodies, an organic electroluminescence element (also referred to as organic EL element hereinafter), which uses an organic material to emit light by electric energy from a positive electrode and a negative electrode, has particularly received attention because it can emit light at a low voltage of several volts to several tens volts, is a thin-film type completely-solid stare element, can save space, and the like.
In order to increase the efficiency of the organic EL element, it is necessary to increase light extraction efficiency; however, in the organic EL element, since the distance between a light emitting layer and a metal electrode is small at a level of about several tens nm, waveguide loss of surface plasmon mode light is large, said therefore the light extraction efficiency does not increase. A known method to reduce the waveguide loss of the surface plasmon mode light is to configure the element as a top emission type organic EL element in which the anode is a reflecting electrode.
A metal thin-film is used as the cathode of the top emission type organic EL element, whereon the metal thin-film is thinned to a degree at which electrical conductivity is not damaged. However, if the metal thin-film has a thickness that ensures sufficient electrical conductivity, transmission of the cathode will decrease, and therefore extraction efficiency of the light emitted from inside will be reduced; further, light entering the cathode at a given angle will be more strongly reflected by an interface between the cathode and the organic layer, and therefore variation in brightness of the light-emitting element with the viewing angle will become larger.
Patent document 1 discusses a method to improve the light extraction efficiency by using optical interference; however, the improvement of light extraction efficiency achieved by using optical interference provides no freedom degree in film thickness of light emitting layer, nor freedom degree in the distance between a reflecting electrode and a semi-transmissive electrode, and therefore it is difficult to ensure compatibility between the light extraction efficiency and other performances such as voltage, lifetime and the like.
Further, Patent document 2 discusses a method to improve the light extraction efficiency by providing a light scattering layer on the outer side of a transmissive electrode; however, the problem with such a method is that it is not possible to avoid optical loss caused by back scatter of the light scattering layer, and that, in the case of a dual emission type organic EL lighting, transmission of the element will decrease due to provision of the light scattering layer.